1. Technical Field
This invention relates generally to methods and apparatus for measuring the composition, velocity and volumetric flow rate of a multi-phase fluid flow (e.g., oil, water, and gas mixture), and more particularly to methods and apparatus for measuring the liquid component portion of a multi-phase fluid flow.
2. Background Information
Currently, there is an unmet need for cost-effective multi-phase fluid flow measurement in oil and gas production. Oil wells very often produce widely varying amounts of oil, water and gas and thus, exhibit a wide range of multi-phase flow patterns. To obtain accurate measurements of the fluid flow mixture, current methods typically separate the produced fluid into component phases prior to measurement. For example, producers have historically relied on three-phase separators to divide the production streams into single-phase oil, water and gas streams for measurement using conventional, single-phase flow meters. Although generally effective, three-phase separators have several undesirable properties that have driven the industry to seek alternative solutions, including size, cost and limited turndown ratios. Recently, advancement of online water cut and gas/liquid separation technology has enabled the industry to consider compact approaches based on two-phase separation. In these systems, the fluid flow is separated into a gas component flow and liquid component flow for measurement and the net oil is determined by measuring the liquid rate and water cut of the liquid component flow. Most of these systems strive to completely separate all of the liquid into the liquid component stream (as opposed to capturing only a portion of the liquid component flow), and all of the gas into the gas component flow.
One of the difficulties in measuring the composition of a fluid flow at a well head is that the pipe is often not completely and continuously filled with liquid; e.g., the gas void fraction may randomly vary from 0% to 100% of the flow. Currently available apparatus for measuring the water cut has difficulty or is unable to accurately measure the water cut of the fluid due to factors such as intermittent liquid flow.
What is needed is a cost effective apparatus and method for multi-phase fluid flow measurement in oil and gas production, and one that is operable to determine the water cut of a fluid flowing within a pipe under variable flow conditions, including flow conditions of less than 100% liquid within the pipe.